In wind turbines, tower vibrations will be excited by wind conditions in the wind field acting on the wind turbine. In land based wind turbines these tower vibrations are of high frequency and produce only small tower inclinations. However, e.g. wind turbines mounted on floating foundations of the so-called spar-buoy type can have, due to wind conditions, low vibrational frequencies of the tower and large inclinations of the tower and therefore large movements of the nacelle.
A combination of low tower frequency and negative thrust curve at high wind speeds can lead to high tower loads and large inclination variations of the nacelle if the turbine is constant power controlled. In constant power control the wind turbine is usually controlled to maintain approximately a constant rotor speed and the torque of the rotor is regulated by setting an appropriate pitch angle of the turbine blades so as to maintain an approximately constant power. High tower loads and large inclination variations in this power control mode may lead to instabilities and structural failure of the wind turbine. Until now, therefore, wind turbines have not been installed on towers with very low tower frequency (below 0.05 Hz).
In U.S. Pat. No. 6,891,280 B2 a method for operating off-shore wind turbine plants based on the frequency of their towers is disclosed. In this method, it is avoided to operate the plant in a frequency range in which the resonant tower frequency lies. To achieve this, the critical natural frequency of the plant and the speed of the rotor where the plant is excited in its critical natural frequency range are determined. Then, this speed range is avoided during operation of the wind tower plant by operating above or below said critical speed range and, if necessary, by rapidly passing through the critical speed range.
A method of damping tower vibrations of a wind turbine is disclosed in WO 2007/053031 A1. In this method, tower Eigen vibrations are damped by an angular increment Δβ which is added to the blade pitch angle. This increment is calculated on the basis of the tower velocities, in order to counteract the Eigen vibrations.
A method for damping vibrations of a wind turbine tower is disclosed in US 2006/0066111 A1. In this method the generator demand torque is modulated by providing a signal based on changes in the generator rotor speed.